In the image forming apparatus to form an image by transforming the toner image onto the sheet, it is known that the sheet extends and contracts by effects of heat and pressure, and the size of the image formed on the sheet changes due to fluctuation of amounts of extension and contraction of the sheet in accordance with types of the sheets used.
There is widely used a method to change a moving speed of the image carrier as a method to adjust the magnification (hereinafter called a longitudinal magnification) of the image in a sheet conveyance direction. Namely, the longitudinal magnification is changed by changing the moving speed of the image carrier with respect to scan exposing carried out by a laser diode array or a light emitting diode array in a constant speed in a main scanning direction (a direction perpendicular to the moving direction of the image carrier) and in a sub-scanning direction (the moving direction of the image carrier).
In order to transfer the image formed on the image carrier onto the recording sheet, at a transfer position the recording sheet has to be conveyed in the same speed as that of the image carrier thus in order to change the lateral magnification the recording sheet conveyance speed is changed beside the image carrier moving speed.
Also, in order to match the sheet conveyance speed at a transfer position coincides and the image carrier moving speed, the conveyance speed of the sheet feeding section to feed the sheet to the transfer position is adjusted finely. As a driving section to drive the sheet feeding section, a stepping motor is usually used. By adjusting a frequency of a drive clock pulse to drive the stepping motor, fine adjustment of the conveyance speed of the sheet feeding section is conducted.
In Patent Document 1: Unexamined Japanese Patent Application Publication No. 2009-29003, there is a contraption that a condition not to lose steps of PLL is stored and read so that a rotation speed of a polygon mirror to conduct exposure in the main scanning direction is changed in a minimum amount of time required.
In Patent Document 3: Unexamined Japanese Patent Application Publication No. 2002-2039, besides changing a rotation speed of a photoconductive member, by changing a rotation speed of a register roller in accordance with the change of the rotation speed of the photoconductive member, the longitudinal magnification is changed.
Patent Document 1: Unexamined Japanese Patent Application Publication No. 2009-29003
Patent Document 3: Unexamined Japanese Patent Application Publication No. 2002-2039
As described above, to change the longitudinal magnification, the rotation speed of the polygon mirror and the photoconductive drum is changed and the moving speed of the intermediate transfer belt is changed. However it is known that the above changes are time consuming and decrease productivity. In particular, in a color image forming apparatus utilizing an intermediate transfer method, in order to match the obverse and reverse magnifications, when adjusting the obverse and reverse magnifications, adjusting periods are inserted between every obverse side and reverse side printing which drastically decrease the productivity.
Also, in case the intermediate transfer member and the secondary transfer member are in direct contact or pressed each other having the sheet in between without slippage, if the drive speed of the secondary transfer member is changed, there is a problem that the drive of the intermediate transfer member is interfered and misalignment of transfer and color shift occur.